1. Field of the Invention
This invention relates generally to a sound image processing for positioning audio signals being reproduced over headphones and, more particularly, to apparatus for panning the apparent sound location so that it appears to move relative to the listener with smooth transitions during the panning operation.
2. Description of the Background
Although a number of schemes have been proposed for providing sound image positioning during playback over two or more loudspeakers, only recently has such sound image positioning been proposed for headphone use. Different sound processing techniques are required for the headphone reproduction in view of the human hearing mechanism.
One approach for providing accurate sound image placement during reproduction over headphones is disclosed in U.S. patent application Ser. No. 08/069,870 filed Jun. 1, 1993 and assigned to the assignee of this application. The disclosure in the above-identified application is incorporated herein by reference. In the system of that application, front and back sound location filters are provided, and it is possible to pan from left to right through 180.degree. using the front filter and then from right to left through 180.degree. using the rear filter. A number of scalers are provided at the filter inputs and/or outputs to adjust range and location of the apparent sound source. Thus, a large number of circuit components and filtering power is required to provide a realistic sound image placement and to provide movement of the sound image location using the front and back filters, a pair of which are required for the left and right ears. In addition, in the previously proposed system, there is a certain granularity in the sound as the sound position is stepped around the various azimuth locations relative to the listener. Such sound position increments can sometimes be heard and are annoying. Thus, previous systems do not permit a smooth fading between the various sound origin positions.
Therefore, a sound positioning system for use with headphones that can provide a smooth transition between locations as the apparent sound location is panned relative to the listener is quite desirable. In addition, a system with a reduced number of components but can still provide capability for panning a number of channels or voices is highly desirable.